Pavement design is a process intended to find the most economical combination of layer thickness and material type for the pavement, taking into account the properties of the subgrade and the traffic to be carried during the service life of the road. Low traffic road pavements with a thin bituminous surfacing and granular base and subbase layers represent, in France, about 60% of the road network. Recent studies dealing with the improvement of design methods of flexible pavements have pointed out that in most cases, when the bearing capacity of the soil is sufficient, this rutting takes place mainly in the granular base and subbase layers. Moreover, fatigue or cracking of the thin bituminous wearing course generally appears much later, when significant rutting has already developed. Pavement design methods for low traffic pavements, used in pavement mechanics, are based on linear elastic calculations. In the French design method, two design criteria are used to determine pavement life : a fatigue criterion for the asphalt layer, and a rutting criterion for the subgrade. The rutting criterion is very simplified, and consists in limiting the vertical elastic strains at the top of the subgrade. This criterion is based on the observation of the performance of existing pavements, and not on any laboratory characterisation of the materials. No criterion is presently applied for the rutting of the unbound granular layers and no model is used to predict the permanent deformations due to cyclic loading.